1. Field of the Invention
The present invention relates to implantable medical devices and methods, and more particularly, to an implantable dual-chamber pacemaker that automatically adapts its atrioventricular (AV) delay, or AV interval, in order to avoid fusion beats.
The heart is a pump that pumps life-sustaining blood throughout the body of the patient. The human heart comprises a left side and a right side with each side having a first chamber known as the atrium, and a second chamber known as the ventricle. The atrium receives blood returning from other body locations. At an appropriate time, determined by the sinoatrial (SA) node, an electrical stimulus is provided that causes the muscle tissue surrounding the atrium to depolarize. Depolarization of the atrial muscle tissue is manifest by the occurrence of an electrical signal known as the P-wave. Immediately following the P-wave, the atrial muscle tissue physically contracts, forcing the blood held in the atrium through a one-way valve into the ventricle. The SA node stimulus that caused the atrium to depolarize also travels to the ventricle through the atrioventricular (AV) node and the atrioventricular (AV) bundle. The AV node is a mass of modified heart muscle situated in the lower middle part of the right atrium. It receives the impulse to contract from the sinoatrial node, via the atria, and transmits it through the atrioventricular bundle to the ventricles. The AV bundle is a bundle of modified heart muscle fibers (Purkinje fibers) that pass from the AV node forward to the septum between the ventricles, where it divides into right and left bundles, one for each ventricle. The fibers thus transmit the SA node stimulus from the atria, via the AV node, to the ventricles. However, as the SA node stimulus travels through the AV bundle, it is delayed by an amount commensurate with the same time it should take the blood to physically flow from the atrium to the ventricle.
After the delay through the AV bundle, which delay is referred to herein as the "natural conduction time" of the heart, the SA node stimulus arrives at the ventricular muscle tissue, causing it to depolarize. Depolarization of the ventricular muscle tissue is manifest by the occurrence of an electrical signal known as the R-wave (sometimes referred to as the QRS complex). Immediately following the R-wave, the ventricular muscle tissue physically contracts, forcing the blood held therein through one or more arteries to various body locations. In this manner, then, the heart "beats" or pumps blood by having the atria contract at a rate determined by the SA node, and after the natural conduction time, by having the ventricles contract. After a longer delay, when the atrium has refilled with blood returning from throughout the body, the process repeats.
The heart of a typical healthy patient may beat 60-70 times per minute when the patient is at rest. When the patient is undergoing significant physiological stress, as occurs, e.g., during physical exercise, the rate at which the heart beats, the "heart rate," increases significantly, e.g, up to 150-170 times per minute. The above-described process wherein the atria and ventricles sequentially depolarize and contract in order to pump blood and get ready to depolarize again, is referred to herein as the "cardiac cycle." A given cardiac cycle thus includes one R-wave (or equivalent ventricular activity evidencing depolarization of the ventricles) and one P-wave (or equivalent atrial activity evidencing depolarization of the atria). The length of the cardiac cycle (which represents the period of the heart rate) may be measured as the time interval between successive P-waves or R-waves, although R-waves are usually used because they are easier to detect.
A pacemaker is an implantable medical device that monitors the activity of the heart for the occurrence of P-waves and/or R-waves, and steps in with electronically generated stimuli, when needed, to force the depolarization of the atria and/or ventricles. A pacemaker-generated stimulus that is delivered to the atrium is referred to herein as an "A-pulse." A pacemaker-generated stimulus that is delivered to the ventricle is referred to herein as a "V-pulse." Most pacemakers are configured to provide an A-pulse and/or V-pulse only if a prescribed period of time has elapsed without the occurrence of a P-wave and/or an R-wave, i.e., without the occurrence of natural heart beats.
The prescribed period of time used by the pacemaker between contraction of the ventricle and contraction of the atrium is generally referred to as the V-A Interval, or the atrial escape interval. For most dual-chamber pacemaker modes of operation, only if a P-wave does not occur during the atrial escape interval will the pacemaker step in at the conclusion of such interval and generate an A-pulse.
The prescribed period of time used by the pacemaker between contraction of the atrium and contraction of the ventricle is referred to as the "AV interval," or sometimes it is called the "AV Delay."The pacemaker, for most dual-chamber modes of operation, generates a V-pulse only if the AV Interval elapses without the occurrence of an R-wave.
In the above-described manner, the heart is thus afforded as much time as possible to beat on its own before the electronically-generated stimuli of the pacemaker are delivered to the heart, causing it to beat at the rate set by the pacemaker.
When a pacemaker is first implanted in a patient, or thereafter, the value of the AV interval is set by an attending physician or cardiologist to a value that is selected to optimally assist the patient's heart as it performs its critical function of a pump. For many patients, such AV interval value is a value that is somewhat longer than the natural conduction time of the heart, thereby affording the patient's heart as long a time period as possible before stepping in with the pacemaker generated stimulation pulse (V-pulse). Such action further serves to lengthen the battery life of the pacemaker, because it reduces the number of stimulation pulses that the pacemaker generates, and thereby conserves the limited energy available in the pacemaker battery. However, for other patients, it may be desirable to set the AV interval value at a value that is less than the natural conduction time of the heart, thereby assuring that a V-pulse is generated with most every cardiac cycle. Such patients typically suffer from a cardiomyopathy condition, and the repeated stimulus improves the ability of the ventricular tissue to produce an effective contraction. See Applicant's copending application entitled IMPLANTABLE PACEMAKER HAVING ADAPTIVE AV INTERVAL FOR PROVIDING VENTRICULAR PACING, Ser. No. 07/975,747, filed Nov. 13, 1992, which application is incorporated herein by reference.
Unfortunately, while the AV interval of a pacemaker can be programmably set to a desired value, the natural conduction time of the patient may vary, either with time, or with the medical or physiological condition of the patient. For example, the natural conduction time may vary as a function of whether the patient is undergoing physiological stress (e.g., exercise), or whether the patient is under the influence of medication. In most instances, it is desirable to have the AV interval closely mimic the natural conduction time, because such natural conduction time normally represents the optimum timing between depolarization of the atria and depolarization of the ventricles. However, when the natural conduction time is varying, it is not possible for the AV interval of the pacemaker to mimic such time. What is needed, therefore, is an implantable pacemaker that automatically adjusts its AV interval to a value that tracks or mimics changes in the natural conduction time.
However, it is important to note that the AV interval cannot be set to the same value as the natural conduction time, else the V-pulse will be generated at the same time that the R-wave occurs, a condition known as "fusion." Fusion is not necessarily harmful to the heart, but it represents the expenditure of wasted energy, as the cardiac tissue is not capable of responding to the V-pulse stimulus when it is refractory. The cardiac tissue is refractory concurrent with and/or immediately following depolarization, and remains refractory until the occurrence of the T-wave. Thus, in order to conserve the limited energy of the pacemaker, it is important that fusion be avoided, and that the V-pulse not be applied to the cardiac tissue while it is refractory, i.e., concurrent with and/or immediately following the occurrence of an R-wave. However, if the natural conduction time varies, as it does, it is quite probable that the natural conduction time will wander into the AV interval time, causing fusion to occur. Thus, what is needed is not only an implantable pacemaker that automatically adjusts its AV interval to track or mimic the natural conduction time, but that adjusts the AV interval to a value that is close to, but not the same as, the natural conduction time, thereby providing the desired tracking while avoiding fusion with the natural depolarization of the patient's heart.
Pacemakers are known in the art that automatically adjust various timing intervals, usually the pacemaker-defined atrial escape interval so as to vary the pacing rate to best suit the sensed physiological needs of the patient. However, care must be exercised when the timing intervals of the pacemaker are varied to prevent the development of cardiac arrhythmias, many of which are actually sustained by the adjustable timing provisions of the pacemaker. It would generally be preferable that a timing change be made quickly, and that it be checked and readjusted often, so as to avoid triggering or sustaining cardiac arrhythmias. What is needed, therefore, is a pacemaker that will not only quickly search for and find the desired AV interval value, but that will automatically detect when an adjustment of the AV interval is needed, or not needed. It would also be desirable if the adjustment of the AV interval could be automatically suspended if it was determined that the adjustment was somehow detrimental to the patient or was occurring at a too frequent rate.